My periods fluctuate between three and seven days, but I haven't noticed much of a change in when I ovulate or the length of the cycle. One possible cause of a lighter, shorter period is that the body didn't produce the normal amounts of progesterone during that cycle, which is the hormone associated with thickening the uterine lining. You shouldn't be too concerned, but if it becomes a pattern and you're trying to become pregnant, a progesterone supplement may be necessary for successful implantation.
From what you described, this period wasn't any different than you last cycle. According to your calculations this period cycle falls correctly with your 3-4 day range. Maybe it was the flow (they amount you bled) of your period that peaked your curiosity. Every cycle will be different when it comes to flow. I don't think you have anything to be concerned about:-)